1. Field of the Invention
This invention relates generally to semiconductor processing and, more particularly, to adjusting processing speeds based on work-in-process levels during semiconductor processing.
2. Description of the Related Art
To fabricate a semiconductor device, a wafer is typically processed through numerous processing tools in a predetermined sequence. The processing tools may include photolithography steppers, etch tools, deposition tools, polishing tools, rapid thermal anneal tools, ion implantation tools, and the like. Each processing tool modifies the wafer according to a particular operating recipe. For example, a photolithography stepper may be used to form a patterned layer of photoresist above the wafer. Features in the patterned layer of photoresist correspond to a plurality of features, e.g. gate electrode structures, which will ultimately be formed above the surface of the wafer. For example, an etching tool may be used to etch portions of the patterned layer to form features, e.g. portions of gate electrode structures, in the patterned layer.
The processing tool sequence, as well as the recipes used by the processing tools, must be carefully controlled so that the features formed on the wafer meet appropriate design and performance criteria. Thus, advanced process control (APC) systems are often used to coordinate operation of the processing tools. However, the processing tool recipes are typically predetermined and provided to the processing tools before they begin processing wafers. For example, the operating recipe may indicate a processing speed associated with the processing tool, e.g., the number of wafers or wafer lots that may be processed in the processing tool in a given time period. The quality of the products formed by the processing tool may be reduced when the predetermined recipe is not changed in response to changing conditions in the fabrication facility. For example, the accuracy or quality of the processing performed by the processing tool is generally inversely related to the processing speed of the processing tool. The predetermined recipe may therefore be selected so that the processing tool may process a relatively large number of wafers or wafer lots at a relatively low, but acceptable, level of accuracy. However, if changing factory conditions result in a smaller than predicted number of wafers or wafer lots being provided to the processing tool for processing, these wafers or wafer lots may still be processed at the relatively high speed and relatively low level of accuracy.